1. Field of the Invention
The present invention generally concerns encapsulating semiconductor devices and specifically an injection casting system for encapsulating semiconductor devices.
2. Related Information
Encapsulating semiconductors is an important step for protecting a semiconductor surface such as a printed circuit board (PCB) from degrading. In addition, encapsulating permits the forming of lens for use with Infrared Data Transceivers (IrDT) while still protecting the PCB.
The use of IrDT as a device to transmit and receive data wirelessly has been gaining popularity. Devices are equipped with infrared modules such as personal digital assistants (PDA), notebook/laptop computers, digital cameras, mobile phones, and many more.
Traditionally, most semiconductor devices use leadframes as substrate materials, but recently PCBs have been widely used to replace leadframes. Some advantages of using a PCB over leadframes are as follows: 1) better coplanarity, 2) easier to scale down, 3) better electromagnetic interference immunity and improved thermal dissipation, and 4) flexible singulation process (sawing over trim/form). These advantages translate into lower investment for equipment. Therefore, an objective of the present invention is the ability to incorporate PCBs into the injection casting system.
Because of its simplicity, low cost, and fast turn-around time, a cast process is extensively used in the encapsulation process. The cast process is especially economical for low volume production and research and development (R&D) work when compared to a transfer mold. Transfer molding onto PCB substrates with an unfilled epoxy also presents more challenges than transfer molding onto leadframes substrates. Accordingly, a further objective of the present invention is to integrate the advantages of the cast process including simplicity, low cost, and fast turn around time.
The current cast process uses room temperature vulcanizing (RTV) silicone as the mold material. RTV silicone comes in two parts which are then mixed and cured in a master mold to form the epoxy casting RTV silicone molds. The resulting RTV silicone molds are then used to encapsulate the semiconductor substrates. The RTV silicone molds are problematic because they wear out fast and normally last for 10 to 20 cast cycles after which new molds will have to be made again. Also, the RTV silicone mold's properties depend on mix ratio and cure temperature and are therefore susceptible to unintended variations. Furthermore, the use of molds using silicone are problematic because silicone absorbs moisture which can create problematic air bubbles. Silicone is not the best processing agent either because of long preheat and cure time because silicone has poor thermal conductivity. Therefore, a further objective of the present invention is to avoid the problems associated with RTV silicone molds, minimize mold wear, minimize water and air absorption, and provide for shorter preheat and cure times.
In the prior art, once the RTV silicone molds were formed an operator was required to fill each individual cavity with epoxy, each cavity may have lens aperture which would also have to be individually filled. This process is manual and requires a long cycle time. In addition, the yield varies depending on the operator's skill. An automatic solution for this process is difficult to achieve because of the individual manipulation of epoxy for each cavity and lens aperture. Therefore, a still further objective of the present invention is the provision of a fast, simple and easy process that shortens cycle time and improves yield.
Traditionally, after applying the epoxy, the PCB is placed onto the mold. This process results in a high probability of air entrapment that can not be removed. This step is especially problematic because neither a vacuum chamber nor a pressurized chamber may be used to prevent air entrapment. Accordingly, a still further objective of the present invention is the use of a vacuum or pressurized chamber to eliminate air bubbles.
In addition, several other problems are identifiable with RTV silicone molds. For example, the substrate or PCB size is limited due to a large thermal expansion mismatch between silicone RTV and PCB substrates and the narrow processing (i.e. curing) temperature for RTV silicone molds. Therefore, a still further objective of the present invention is to minimize the expansion mismatch and the narrow processing temperature associated with RTV silicone molds.
A still further objective of the present invention is the provision of a injection casting system for encapsulating semiconductor devices and method of use that is economical to manufacture, durable in use, and efficient in operation.